This invention relates to a method of enhancing pit replication in the molding of optical disks. More particularly the method relates to chemical modification of polycarbonate endgroups which results in improved molding properties and in particular improved pit replication.
Polycarbonates, especially aromatic polycarbonates, are routinely used in the manufacture of optical disks and are prized for their transparency, toughness, and processability. Bisphenol A polycarbonate is widely used in the fabrication of optical data recording media, including optical disks as exemplified by compact audio disks, CD-ROM disks, and digital versatile disks (DVD).
As data storage densities are increased, physical features of the optical disks are compressed. Thus, as data storage density is increased, features of the disk such as tracks or grooves must be decreased in size in order to accommodate an increased number of these features per unit area of the disk. As the size a physical feature of the disk, for example the tracks, is decreased, the controlled reproduction of this physical feature, referred to generally as pit replication, becomes increasingly difficult. In DVD-R disks, for example, information is encoded in a recordable dye at the bottom of a track which has been molded into a polycarbonate substrate. A laser reading or writing to a DVD-R disk which encounters a defective portion of the track (a portion of the track which does not conform specified track dimensions) will skip to a properly dimensioned portion of the track thereby decreasing the data storage capacity of the disk. This xe2x80x9claser skippingxe2x80x9d results in xe2x80x9cblock errorxe2x80x9d and a corresponding xe2x80x9cblock error ratexe2x80x9d. There is a direct relationship between xe2x80x9cpercent replicationxe2x80x9d of a molded-in physical feature such as the disk track and the block error rate. The higher the percent replication value (the closer the pit replication value is to 100 percent) the lower will be the block error rate. Thus, the ability to reproducibly mold a feature such as a disk track of specified dimensions into an optical data storage device such as a DVD-R disk is keenly sought after by commercial manufacturers of optical data storage devices.
Although polycarbonate is widely employed in the manufacture of optical devices such as optical disks, discovery of methods for further improving the utility of polycarbonates by enhancing pit replication in optical disks molded from polycarbonates represents an attractive goal from both technical and commercial perspectives. It is of interest, therefore, to develop methods for enhancing pit replication in the molding of optical disks comprising polycarbonate.
The present invention provides a method for enhancing pit replication in the molding of optical data storage devices comprising polycarbonate. This and further objects of the invention will be more readily appreciated when considering the following disclosure and appended claims.
In one aspect the present invention relates to a method of enhancing pit replication in the molding of optical data storage devices comprising one or more polycarbonates, said method comprising capping at least one of said polycarbonates with end groups having structure I 
wherein R1 is a C14-C30 alkyl group, R2 is a C1-C30 alkyl group, n is an integer from 1 to 5 and m is an integer from 0 to 4.
In another aspect the present invention provides a method for the enhancement of pit replication in the molding of optical data storage devices comprising two or more constituent polycarbonates by capping at least one of the constituent polycarbonates with endgroups having structure I in a polymerization step or in a post polymerization step.
The present invention may be understood more readily by reference to the following detailed description of preferred embodiments of the invention and the examples included herein. In this specification and in the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings.
The term xe2x80x9cpolycarbonatexe2x80x9d as used herein includes copolycarbonates, homopolycarbonates and (co)polyestercarbonates.
The terms xe2x80x9cendcapping agentxe2x80x9d and xe2x80x9cchainstopping agentxe2x80x9d are used interchangeably.
The term xe2x80x9cshort chain alkylphenolxe2x80x9d as used herein refers to an alkylphenol in which the total number of alkyl group carbon atoms attached to the phenolic ring is less than 14. The total number of alkyl group carbon atoms attached to the phenolic ring is referred to as the xe2x80x9ccarbon countxe2x80x9d. Short chain alkylphenols are exemplified by p-cresol and o-cresol (carbon count=1); 2,6-dimethylphenol (carbon count=2); 4-octylphenol (carbon count=8); 4-nonylphenol (carbon count=9), 4-dodecylphenol (carbon count=12); 2-methyl-4-decylphenol (carbon count=11); 2,6-dimthehyl-3-undecylphenol (carbon count=13).
The term xe2x80x9clong chain alkylphenolxe2x80x9d as used herein refers to an alkylphenol in which the total number of alkyl group carbon atoms attached to the phenolic ring is greater than 14. The total number of alkyl group carbon atoms attached to the phenolic ring is referred to as the xe2x80x9ccarbon countxe2x80x9d. Long chain alkyl phenols are exemplified by 4-pentdecylphenol and 3-pentadecylphenol (carbon count=15); 2,6-dimethyl-4-tetradecylphenol (carbon count=16); 2,4-dioctylphenol (carbon count=16); and 2,4-dinonylphenol (carbon count=18).
As used herein the terms 3-pentadecylphenol, meta-pentadecylphenol, and cardanol are used interchangeably.
As used herein the term xe2x80x9cpit replicationxe2x80x9d refers to the process wherein the features of a mold stamper are transferred to a thermoplastic material comprising at least one polycarbonate during the molding of an optical data storage device, such as an optical disk. Thus, the term refers to the replication of molded in features such as grooves or tracks as well as pits, and includes instances in which the feature being molded into the thermoplastic does not include pits. With reference to optical disks, the term pit replication as defined herein encompasses the replication of disk features generally.
As used herein the term xe2x80x9cpit replicationxe2x80x9d encompasses the terms xe2x80x9cgroove replicationxe2x80x9d and xe2x80x9ctrack replicationxe2x80x9d when used in reference to the molding of an optical disk.
As used herein the term xe2x80x9cpercent replicationxe2x80x9d is a comparison of the difference in dimensions between the features replicated in a thermoplastic by a stamper during molding, and the physical features of the stamper itself, wherein both the dimensions of the replicated features in the thermoplastic and those of the stamper are determined by Atomic Force Microscopy (AFM). For example, in a molded optical disk comprising polycarbonate one may measure the dimensions of a disk track located 55 mm from the disk center using AFM and compare said dimensions with the corresponding physical feature on the stamper. The measured depth of the molded track is divided by the value of the analogous physical feature on the stamper and is multiplied by 100 to give the percent replication value.
As used herein the terms xe2x80x9cpercent pit replicationxe2x80x9d and xe2x80x9cpercent replicationxe2x80x9d are used interchangeably.
As used herein the terms xe2x80x9ccappingxe2x80x9d and xe2x80x9cendcappingxe2x80x9d are used interchangeably.
The present invention provides a method for enhancing pit replication in the molding of optical data storage devices, such as optical disks, comprising one or more polycarbonates, said method comprising a step of capping at least one of the constituent polycarbonates with end groups having structure I. The one or more polycarbonates used in the molding of the optical disks comprise at least one repeat unit corresponding to structure II 
wherein R3-R10 are independently a hydrogen atom, halogen atom, nitro group, cyano group, C1-C20 alkyl radical C4-C20 cycloalkyl radical, or C6-C20 aryl radical; W is a bond, an oxygen atom, a sulfur atom, a SO2 group, a C1-C20 aliphatic radical, a C6-C20 aromatic radical, a C6-C20 cycloaliphatic radical or the group 
wherein R11 and R12 are independently a hydrogen atom, C1-C20 alkyl radical, C4-C20 cycloalkyl radical, or C4-C20 aryl radical; or R11 and R12 together form a C4-C20 cycloaliphatic ring which is optionally substituted by one or more C1-C20 alkyl, C6-C20 aryl, C5-C21 aralkyl, C5-C20 cycloalkyl groups or a combination thereof.
Polycarbonates comprising repeat units II may be prepared by polymerizing one or more bisphenols III 
wherein R3-R10 and W are defined as in strucutre II, with a source of carbonate units such as phosgene or a diaryl carbonate.
In one embodiment of the present invention at least one polycarbonate comprises repeat units IV, said repeat units being derived from bisphenol A. 
The method of the present invention may be practiced by performing the polycarbonate capping during a polycarbonate polymerization step, or in a post polymerization step. The capping carried out during a polymerization step may be performed under a variety of conditions. In one embodiment of the present invention the capping of the polycarbonate is carried out conveniently in a conventional interfacial reaction of phosgene with at least one bisphenol III, in which a long chain alkylphenol V 
wherein R1 is a C14-C30 alkyl group, R2 is a C1-C30 alkyl group, n is an integer from 1 to 5, and m is an integer from 0 to 4 is employed as a capping agent. The conditions of such reactions are well known in the art; they include the use of a mixture of water and a water-immiscible organic liquid such as methylene chloride as a reaction medium; the presence of a tertiary amine such as triethylamine or dimethylbutylamine, the optional presence of a phase transfer catalyst such as tetrabutylammonium chloride or hexaethylguanidinium chloride; and the presence of a water soluble metal hydroxide such as sodium hydroxide as an acid acceptor. Typical proportions of capping agent are in the range of about 0.1-10 mole percent based on the total amount of bisphenol employed. Suitable bisphenols III include 2,2-bis(4-hydroxyphenyl)propane; 2,2-bis(3-methyl-4-hydroxyphenyl)propane; 1,1-bis(3-methyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(4-hydroxyphenyl)cyclohexane; and 1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane. Suitable long chain alkylphenols V include meta-pentadecylphenol; 4-octadecylphenol, 3-octadecylphenol, 2-octadecylphenol, and mixtures thereof; 2-methyl-3-pentadecylphenol and the like.
In an alternate embodiment of the present invention the capping of the polycarbonate is carried under melt polymerization conditions. The term xe2x80x9cmelt polymerization conditionsxe2x80x9d will be understood to mean those conditions necessary to effect reaction between a diaryl carbonate and a dihydroxy aromatic compound to produce a polycarbonate. The reaction temperature is typically in the range of about 100 to about 350xc2x0 C., more preferably about 180 to about 310xc2x0 C. and typically a reaction vessel adapted for the removal of volatile by-products is employed. The pressure may be at atmospheric pressure, supraatmospheric pressure, or a range of pressures from atmospheric pressure to about 15 torr in the initial stages of the reaction, and at a reduced pressure at later stages, for example in the range of about 0.001 to about 15 torr. The reaction time is generally about 0.1 hours to about 10 hours.
Typically, where the capping is to be carried out during the melt polymerization of polycarbonate, one or more bisphenols III are reacted with at least one diaryl carbonate in the presence of a melt polymerization catalyst, optionally a co-catalyst, and at least one long chain alkyl phenol V at a temperature in a range between about 100xc2x0 C. and about 350xc2x0 C., preferably between about 180xc2x0 C. and about 310xc2x0 C., and a pressure in a range between ambient pressure and about 0.001 mmHg. Melt polymerization catalysts which may be employed include alkali metal hydroxides such as sodium hydroxide, or metal salts of polyacids such as ethylenediamine tetraacetic acid magnesium disodium salt, in an amount corresponding to between 1xc3x9710xe2x88x928 and about 1xc3x9710xe2x88x923 moles catalyst per total moles of bisphenol employed. The melt polymerization catalyst may further comprise one or more cocatalysts such as a tetraalkylammonium salt or a tetraalkylphophonium salt. Where such cocatalysts are employed, they are typically present in an amount corresponding to between about 1 and about 1000 times the amount of alkali metal hydroxide catalyst employed. Tetraalkylammonium salts suitable for use as cocatalysts include tetramethylammonium hydroxide and tetrabutylammonium hydroxide. Tetraalkylphosphonium salts suitable for use as co-catalysts include tetrabutylphosphonium acetate and tetrabutylphosphonium hydroxide. Suitable bisphenols III include 2,2-bis(4-hydroxyphenyl)propane; 2,2-bis(3-methyl-4-hydroxyphenyl)propane; 1,1-bis(3-methyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(4-hydroxyphenyl)cyclohexane; and 1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane. Suitable long chain alkylphenols V include meta-pentadecylphenol; 4-octadecylphenol, 3-octadecylphenol, 2-octadecylphenol, and mixtures thereof; 2-methyl-3-pentadecylphenol and the like. Suitable diaryl carbonates are illustrated by diphenyl carbonate, dinaphthyl carbonate, bis(2-chlorophenyl)carbonate, bis(ethyl salicyl) carbonate, bis(propyl salicyl) carbonate, bis(phenyl salicyl) carbonate, bis(benzyl salicyl) carbonate, bis(methyl salicyl) carbonate, and the like.
In yet an alternate embodiment of the present invention capping may be carried out on polycarbonate in a post-polymerization step. Typically, polycarbonates which may be capped in a post-polymerization step have number average molecular weights (Mn, as measured by gel permeation chromatography using polystyrene molecular weight standards) in a range between about 7,000 and about 25,000 daltons, said polycarbonates having at least about 10 percent, preferably about 20 percent and still more preferably about 50 of the chain terminating groups being hydroxyl groups.
In one embodiment of the present invention a polycarbonate comprising hydroxy terminal groups is treated under xe2x80x9cmelt polymerization conditionsxe2x80x9d with a diaryl carbonate in the presence of a long chain alkylphenol V at a temperature in a range between about 100xc2x0 C. and about 350xc2x0 C., preferably between about 180xc2x0 C. and about 310xc2x0 C. at a pressure between about ambient pressure and about 0.001 mmHg. The employment of a catalyst is optional. Typically, melt polymerization catalysts are effective in promoting post-polymerization capping of polycarbonate. Polycarbonates which may be capped in a post polymerization step include bisphenol A polycarbonate and the like.
In yet another embodiment of the present invention capping may be carried out on polycarbonate in a post-polymerization step wherein a polycarbonate comprising hydroxy terminal groups may be treated under xe2x80x9cmelt polymerization conditionsxe2x80x9d with a diaryl carbonate which is itself derived from one or more alkylphenols V, for example bis (4-octadecylphenyl)carbonate, phenyl 4-octadecylcarbonate, or phenyl 3-pentadecyl carbonate. The temperature at which the capping reaction is carried out is in a range between about 100xc2x0 C. and about 350xc2x0 C., preferably between about 180xc2x0 C. and about 310xc2x0 C. at a pressure between about ambient pressure and about 0.001 mmHg. The employment of a catalyst is optional. Polycarbonates which may be capped under these conditions include bisphenol A polycarbonate and the like.
The method of the present invention provides enhanced xe2x80x9cpit replicationxe2x80x9d in the molding of optical data storage devices comprising polycarbonate. In one embodiment of the present invention the optical data storage device is an optical data storage disk, for example a digital versatile disk (DVD), a recordable digital versatile disk (DVD-R), a DVD-RAM device, a compact disk (CD), a recordable compact disk (CD-R), compact disks with multiple read/write capabilities (CD-R/W) and the like. A typical optical data storage disk such as a DVD-R is comprised of polycarbonate embossed with grooves, sometimes referred to as pits or tracks, on one of its surfaces. These grooves are impressed into the polycarbonate during molding in a step which comprises stamping the polycarbonate at elevated temperature with a stamper, said stamper comprising a series of grooves complimentary to those being embossed upon the disk. The method of the present invention facilitates the reproduction of the stamper surface features, for example grooves. Typically, the features being molded into the disk have dimensions between about 20 and about 250 nanometers, for example an optical data storage disk comprising grooves, said grooves having both depth and width, said depth being between about 20 and about 250 nanometers, and said width being between about 20 and about 250 nanometers.
This fidelity with which a surface feature of a stamper may be molded into the surface of an optical data storage device, such as an optical data storage disk, is reflected in the xe2x80x9cpercent replicationxe2x80x9d or xe2x80x9cpercent pit replicationxe2x80x9d value. It is desirable that the xe2x80x9cpercent pit replication value be at least 80 percent, preferably at least 90 percent, and even more preferably greater than 95 percent.
As mentioned in one embodiment of the present invention at least one polycarbonate comprises bisphenol A-derived repeat units IV is capped to provide a xe2x80x9ccapped polycarbonatexe2x80x9d comprising endgroups having structure I. The physical properties of a polycarbonate so constituted will depend on the extent to which the polycarbonate has been capped. Typically, it has been found most advantageous when molding optical data storage disks, such as DVD-R""s, to limit the number of endgroups comprising structure I to between about 5 and about 95 percent, preferably between about 5 and about 50 percent, and still more preferably between about 5 and about 30 percent of the endgroups present in the polymer. In the case of polycarbonates comprising structural units other than structure IV the optimum level of endgroups comprising structure I may be greater or less than the xe2x80x9cabout 5 to about 50 percentxe2x80x9d range appropriate for BPA homopolycarbonate which possesses, apart from the chain termini, exclusively repeat units having structure IV.